You control at least one nontoken artifact creature.
You control less than four Blood tokens.
Steps
Activate Krark-Clan Ironworks by sacrificing a nontoken artifact creature, adding ({C} magic symbol) ({C} magic symbol) .
When the creature dies, Voldaren Bloodcaster and Nim Deathmantle trigger.
Resolve the Voldaren Bloodcaster trigger, creating a Blood artifact token.
Activate Krark-Clan Ironworks by sacrificing the Blood token, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return the artifact creature from your graveyard to the battlefield, and attach Nim Deathmantle to it.